The present invention relates to the field of synthetic aperture radar (SAR), and in particular to signal processing apparatus for a SAR system having a rotating antenna.
German Patent Document DE-PS 39 22 086 discloses a rotating antenna synthetic aperture radar system (ROSAR Device) in which at least one antenna for transmitting and receiving radar pulses is arranged on the end of a rotating arm (for example, of a helicopter rotor or of a turnstile above the rotor axis). The received signals are demodulated and stored intermediately, and are then correlated with reference functions which are each calculated and preset based on the illumination geometry of the radar device.
The parameters for calculation and presetting of the reference functions are the distance intervals to be measured, the transmission frequency, the length of the rotating arm; the angle of rotation range of the antenna from which the reflected signals are receive; the number of transmitted pulses as well as the height of the rotating antenna above the ground. The correlation result is appropriately displayed, for example, on a monitor.
A radar device of this type may be used in approximate real time, in on-line operation. It thus can be used, for example, not only in the field of cartography and in obstacle warning operations, but also for the purpose of target reconnaissance and tracking. The processor of this known ROSAR-device has several modules so that the multiple and complex computing tasks may be subdivided, thereby permitting the approximate real time and on-line operation.
In this known apparatus, the result for each distance interval is always obtained by correlation or the received signal with a single reference function that is valid for that distance interval.
German Patent Document DE-PS 39 22 086 sets forth an approach for simplifying these reference functions under certain conditions; specifically when the target distance is either significantly larger than the height (or altitude) of the antenna above the ground, and is also much larger (or at least significantly larger) than the length of the rotor arm carrying the antenna. In all other cases, a complete solution (that is, without simplification) of the correlation is calculated. Despite the simplification measures described in this prior art, therefore, there remain considerable computing expenditures which require an extreme coordination of the computing operations.
One object of the present invention therefore, is to provide a method and apparatus for simplifying and reducing the computations necessary to perform the correlation referred to above, without impairing the broad applicability or the resolution of the radar device.
These and other objects and advantages are achieved according to the invention by performing the correlation for certain ranges using simplified reference functions, in a method which may, as it were, be referred to as "unfocussed signal processing." Basically, correlation of the received signals with the applicable reference function as performed in the prior art corresponds to a focussing, in that complete compensation of the cosinusoidal phase course is performed, and thus all received echoes along the synthetic aperture are summed in phase with the received signed. This operation supplies the best possible resolution, but has the above-mentioned disadvantage of high computing expenditures for correlation.
In unfocussed signal processing, only the main portion of the received signal is taken into account when the phase shift is less than .pi./2 for the go-and-return path of the signal run, which corresponds to a phase error of .pi./4 for the one-way case, or to a path difference of .lambda./8. As long as this condition is met, no correction need be carried out according to the Fraunhofer region condition. Although lateral resolution will deteriorate due to integration along a smaller synthetic aperture length, such degradation is acceptable for certain conditions. The advantage consists of a simplification of the reference function and of the signal processing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.